Wireless communications systems have exhibited remarkable growth over the past decade. Wireless voice and data applications are being enabled by rapidly emerging wireless technologies, such as cellular telephony, personal communications systems, bluetooth and wireless local area networks (WLAN's), to name a few. Digital modulation techniques, miniaturization of transceivers due to advances in monolithic integrated circuit designs and the development of high frequency, microwave and millimeter wave RF systems in both the licensed and unlicensed bands, have all contributed to improving the quality and bandwidth capacity of these systems and to reducing the size and costs of the components.
These systems are having a profound effect on societies. For example, they are enabling service-based economy work forces to become “untethered” from their information sources and conventional wired communications mechanisms. Moreover, throughout the world, wireless communication systems are enabling developing countries to provide instant telephone service to new subscribers who otherwise would have to wait years for wireline access.
Dual band receivers have been introduced to the marketplace that increase the functionality of such communication systems. These receivers can receive only one band at a time and thus must switch between the two different bands. FIG. 1 is a conceptual schematic of such a conventional non-concurent, heterodyne dual band architecture 10. As seen, an incoming signal, Vin, is received at a switch 12 (for simplicity the antenna and front-end filter are not shown). If the signal is in a first predetermined frequency band, ω1, the switch moves to the top signal processing path tuned to match and amplify signals only in this band. The signal is then impedance matched and amplified at low noise amplifier (“LNA”) 20, filtered at band pass filter (“BPF”) 21, mixed with local oscillator signal, LO1, at mixer 22, filtered at BPF2 24 and mixed again with a second local oscillator signal, LO2, at mixer 26, until it exits as Vout1 (e.g. baseband or some low frequency) for further processing (e.g. digital signal processing). If the incoming signal is in the second predetermined frequency band, ω2, the switch 12 moves to the bottom signal processing path tuned to match and amplify signals only in this band. In particular, the signal is amplified by LNA 30, filtered at BPF3 31, mixed with a third local osillator tuned to ω′LO1 at mixer 32, filtered again at BPF4 34 and mixed again with a fourth local oscillator tuned to ω′LO2 at mixer 36 and exists as Vout2. In the example shown, the four oscillators are completely independent devices. While such functionality adds to a device's versatility, such as in the case of a dual-band digital cellular phone, these receivers are very inefficient in terms of component parts and power consumption and would not satisfy the needs for the next-generation of multi-functional devices, such as a cell phone with a GPS receiver and a bluetooth interface.
Another problem with conventional wireless technology relates to bandwidth limitations. The diverse range of modern wireless applications demand wireless communications systems and transceivers with greater bandwidth capacity and flexibility than can be conventionally supplied. Increased bandwidth capacity is necessary for many wireless applications to become a reality. Wireless broadband Internet applications (e.g. browsing, e-commerce, streaming audio and video), wireless video messaging, wireless video games, and remote video monitoring are just a few examples of applications that will be delivered over the next generations of wireless networks. Conventional solid-state radio frequency (“RF” or “wireless”) receiver architectures, such as superheterodyne and direct conversion receivers, accomplish high selectivity and sensitivity by designing them for narrow-band operation at a single RF frequency. Unfortunately, these modes of operation are of limited functionality because they limit the system's available bandwidth and robustness to channel variations. On the other hand, wide-band modes of operation are more sensitive to out-of-band signals due to transistor non-linearity, which can introduce severe bottlenecks in system performance.
Thus, to overcome these and other drawbacks, it would be highly desirable to have a low cost, concurrent dual-band receiver. As used herein a concurrent dual-band receiver is one that can process signals at two discrete frequency bands simultaneously, or substantially simultaneously. This would enable a receiver to significantly increase its bandwidth capacity (bit rate). A concurrent dual-band receiver design could also be used for supplying redundancy in mission critical data transmission application. The reliability of the received signal would be greatly increased with simultaneous transmission of the same signal in multiple bands, because channel properties are different and uncorrelated at two frequency bands and more diversity is achieved.
A further challenge for modem receiver design is to create true concurrent dual-band functionality using as little real estate (and ideally monolithically) and as little power dissipation as possible (and perhaps no more than single band receivers), while keeping the incremental production costs above the conventional single band receiver to a minimum.